This invention relates to retorting oil shale and, more particularly, to a process for retorting oil shale above ground.
Researchers have renewed their efforts to find alternate sources of energy and hydrocarbons in view of past rapid increases in the price of crude oil and natural gas. Much research has been focused on recovering hydrocarbons from solid hydrocarbon-containing material such as oil shale, coal, and tar sands by pyrolysis or upon gasification to convert the solid hydrocarbon-containing material into more readily usable gaseous and liquid hydrocarbons.
Vast natural deposits of oil shale found in the United States and elsewhere contain appreciable quantities of organic matter known as "kerogen" which decomposes upon pyrolysis or distillation to yield oil, gases, and residual carbon. It has been estimated that an equivalent of 7 trillion barrels of oil are contained in oil shale deposits in the United States with almost sixty percent located in the rich Green River oil shale deposits of Colorado, Utah, and Wyoming. The remainder is contained in the leaner Devonian-Mississippian black shale deposits which underlie most of the eastern part of the United States.
As a result of dwindling supplies of petroleum and natural gas, extensive efforts have been directed to develop retorting processes which will economically produce shale oil on a commercial basis from these vast resources.
Generally, oil shale is a fine-grained sedimentary rock stratified in horizontal layers with a variable richness of kerogen content. Kerogen has limited solubility in ordinary solvents and therefore cannot be recovered by extraction. Upon heating oil shale to a sufficient temperature, the kerogen is thermally decomposed to liberate vapors, mist, and liquid droplets of shale oil and light hydrocarbon gases such as methane, ethane, ethene, propane, and propene, as well as other products such as hydrogen, nitrogen, carbon dioxide, carbon monoxide, ammonia, steam, and hydrogen sulfide. A carbon residue typically remains on the retorted shale.
Shale oil is not a naturally occurring product, but is formed by the pyrolysis of kerogen in the oil shale. Crude shale oil, sometimes referred to as "retort oil", is the liquid oil product recovered from the liberated effluent of an oil shale retort. Synthetic crude oil (syncrude) is the upgraded oil product resulting from the hydrogenation of crude shale oil.
The process of pyrolyzing the kerogen in oil shale, known as retorting, to form liberated hydrocarbons can be done in surface retorts in aboveground vessels or in in situ retorts underground. In principle, the retorting of shale and other hydrocarbon-containing materials, such as coal and tar sands, comprises heating the solid hydrocarbon-containing material to an elevated temperature and recovering the vapors and liberated effluent. However, as medium grade oil shale yields approximately 20 to 25 gallons of oil per ton of shale, the expense of materials handling is critical to the economic feasibility of a commercial operation.
In surface retorting, oil shale is mined from the ground, brought to the surface, crushed and placed in vessels where it is contacted with a hot solid heat carrier material, such as hot spent shale, ceramic balls, metal balls, or sand or a gaseous heat carrier material, such as light hydrocarbon gases, for heat transfer. The resulting high temperatures cause shale oil to be liberated from the oil shale leaving a retorted, inorganic material and carbonaceous material such as coke. The carbonaceous material can be burned by contact with oxygen at oxidation temperatures to recover heat and to form a spent oil shale relatively free of carbon. Spent oil shale which has been depleted in carbonaceous material is removed from the retort and recycled as heat carrier material or discarded. The combustion gases are dedusted in cyclones or electrostatic precipitators.
Surface retorting with solid heat carrier material has many advantages over underground retorting and surface retorting with a gaseous heat carrier media. For example, surface retorting with solid heat carrier material produces a substantially greater product yield than underground retorting. Surface retorting with solid heat carrier material attains better heat transfer, more BTVs per volume and greater thermal efficiency than retorting with a gaseous heat carrier media.
The solid heat carrier material should be well mixed with the raw shale to enhance heat exchange and conversion of kerogen to shale oil and light hydrocarbon gases. In the Lurgi-Ruhrgas process, spent shale is mechanically mixed with raw shale in a screw conveyor. In the Tosco II process, ceramic or metal balls (solid heat carrier material) are mechanically mixed with raw shale in a rotating pyrolysis drum. In fluid bed processes, spent shale is fluidly (turbulently) mixed with raw shale in the presence of a pressurized fluidizing gas.
Mechanical mixing utilizes the advantage of surface retorting with solid heat carrier material, but is expensive and suffers from mechanical breakdown and limited throughput capacity.
Fluid bed retorting with solid heat carrier material also offers the advantages of surface retorting but often requires high operating pressures and substantial amounts of fluidizing gas which requires expensive capital outlays for compressors.
Over the years, a number of gravity flow retorts and other retorts have been suggested. Typifying these retorts are those found in U.S. Pat. Nos. 1,432,101; 1,698,345; 1,917,339; 2,624,696; 2,636,263; 2,774,726; 2,894,899; 2,980,617; 3,267,019; 3,281,349; 3,475,317; 3,597,347; 3,703,442; 4,038,045; 4,069,107; 4,087,347; 4,188,184; 4,199,432; 4,211,606; 4,404,086; 4,436,588; and French Pat. No. 756,778. These retorts have met with varying degrees of success.
It is, therefore, desirable to provide an improved retort which overcomes most, if not all, of the preceding problems.